Re: [PATCH v3 08/11] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall

[Cfir Cohen]

In addition, it seems that svm_page_enc_status_hc() accepts 'gpa',
'npages', 'enc' directly from the guest, and so these can take
arbitrary values. A very large 'npages' could lead to an int overflow
in 'gfn_end = gfn_start + npages', making gfn_end < gfn_start. This
could an OOB access in the bitmap. Concrete example: gfn_start = 2,
npages = -1, gfn_end = 2+(-1) = 1, sev_resize_page_enc_bitmap
allocates a bitmap for a single page (new_size=1), __bitmap_set access
offset gfn_end - gfn_start = -1.


On Sun, Jul 21, 2019 at 1:57 PM David Rientjes <rientjes@xxxxxxxxxx> wrote:
>
> On Wed, 10 Jul 2019, Singh, Brijesh wrote:
>
> > diff --git a/Documentation/virtual/kvm/hypercalls.txt b/Documentation/virtual/kvm/hypercalls.txt
> > index da24c138c8d1..94f0611f4d88 100644
> > --- a/Documentation/virtual/kvm/hypercalls.txt
> > +++ b/Documentation/virtual/kvm/hypercalls.txt
> > @@ -141,3 +141,17 @@ a0 corresponds to the APIC ID in the third argument (a2), bit 1
> >  corresponds to the APIC ID a2+1, and so on.
> >
> >  Returns the number of CPUs to which the IPIs were delivered successfully.
> > +
> > +7. KVM_HC_PAGE_ENC_STATUS
> > +-------------------------
> > +Architecture: x86
> > +Status: active
> > +Purpose: Notify the encryption status changes in guest page table (SEV guest)
> > +
> > +a0: the guest physical address of the start page
> > +a1: the number of pages
> > +a2: encryption attribute
> > +
> > +   Where:
> > +     * 1: Encryption attribute is set
> > +     * 0: Encryption attribute is cleared
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 26d1eb83f72a..b463a81dc176 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1199,6 +1199,8 @@ struct kvm_x86_ops {
> >       uint16_t (*nested_get_evmcs_version)(struct kvm_vcpu *vcpu);
> >
> >       bool (*need_emulation_on_page_fault)(struct kvm_vcpu *vcpu);
> > +     int (*page_enc_status_hc)(struct kvm *kvm, unsigned long gpa,
> > +                               unsigned long sz, unsigned long mode);
> >  };
> >
> >  struct kvm_arch_async_pf {
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index 3089942f6630..431718309359 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -135,6 +135,8 @@ struct kvm_sev_info {
> >       int fd;                 /* SEV device fd */
> >       unsigned long pages_locked; /* Number of pages locked */
> >       struct list_head regions_list;  /* List of registered regions */
> > +     unsigned long *page_enc_bmap;
> > +     unsigned long page_enc_bmap_size;
> >  };
> >
> >  struct kvm_svm {
> > @@ -1910,6 +1912,8 @@ static void sev_vm_destroy(struct kvm *kvm)
> >
> >       sev_unbind_asid(kvm, sev->handle);
> >       sev_asid_free(kvm);
> > +
> > +     kvfree(sev->page_enc_bmap);
> >  }
> >
> >  static void avic_vm_destroy(struct kvm *kvm)
>
> Adding Cfir who flagged this kvfree().
>
> Other freeing of sev->page_enc_bmap in this patch also set
> sev->page_enc_bmap_size to 0 and neither set sev->page_enc_bmap to NULL
> after freeing it.
>
> For extra safety, is it possible to sev->page_enc_bmap = NULL anytime the
> bitmap is kvfreed?
>
> > @@ -2084,6 +2088,7 @@ static void avic_set_running(struct kvm_vcpu *vcpu, bool is_run)
> >
> >  static void svm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> >  {
> > +     struct kvm_sev_info *sev = &to_kvm_svm(vcpu->kvm)->sev_info;
> >       struct vcpu_svm *svm = to_svm(vcpu);
> >       u32 dummy;
> >       u32 eax = 1;
> > @@ -2105,6 +2110,12 @@ static void svm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> >
> >       if (kvm_vcpu_apicv_active(vcpu) && !init_event)
> >               avic_update_vapic_bar(svm, APIC_DEFAULT_PHYS_BASE);
> > +
> > +     /* reset the page encryption bitmap */
> > +     if (sev_guest(vcpu->kvm)) {
> > +             kvfree(sev->page_enc_bmap);
> > +             sev->page_enc_bmap_size = 0;
> > +     }
> >  }
> >
> >  static int avic_init_vcpu(struct vcpu_svm *svm)
>
> What is protecting sev->page_enc_bmap and sev->page_enc_bmap_size in calls
> to svm_vcpu_reset()?